1. Field of the Invention
The present invention relates to bar displays, and more specifically, to the production of scale markers therefor.
2. Description of the Prior Art
A bar display device has become known which comprises a flat bar-like gas-filled space having a transparent main anode on its inner front face and a large number of equally spaced cathodes arranged like the rungs of a ladder on the back face of the bar; the cathodes being connected in three interdigitated sets. A "keep-alive" anode, a "keep-alive" cathode, and a reset cathode are arranged at one end of the device. By energizing the reset cathode and then the three main sets of cathodes by a three-phase drive driven by a master clock circuit, a visible gas discharge is produced at one end of the device and then stepped rapidly along the bar from cathode to cathode, appearing as a continuous bar of light. By de-energizing the main anode at a chosen instant, the length of this bar can be controlled.
For a simple display of a variable input quantity, drive circuitry is used comprising means for continuously driving the cathodes with a three-phase drive, a ramp generator synchronized with the cycling of the display, and a comparator which compares the input quantity with the ramp signal and turns the anode voltage off when equality occurs.
This display is self-illuminating, and the use of a normal printed scale on the outside of the display is therefore inconvenient in some circumstances, since the scale will require external illumination to be visible. Further, such a scale must either be on the inside of the display, in which case it is determined for all time when the device is being manufactured, or on the outside, in which case there is the possibility of a parallax error between the display itself and the scale.